The invention concerns a method for obtaining NMR (=nuclear magnetic resonance) spectra of quadrupolar nuclei having spin I>½ using magic angle spinning (=MAS) in solid powders and transfer of coherences from a neighboring nucleus with spin S=½ to single- or double-quantum transitions of quadrupolar nuclei having spin I>½.
A method for obtaining NMR (=nuclear magnetic resonance) spectra of quadrupolar nuclei having spin I>½ is known from [32]-[36].
Although nitrogen is of universal importance in virtually all branches of biology, chemistry, and material science, and although nitrogen-14 is a very abundant isotope (99.4%), nitrogen-14 NMR has enjoyed relatively little popularity so far. In liquids, nitrogen-14 line-widths are very broad because of rapid quadrupolar relaxation, except in rapidly-tumbling molecules or in highly symmetrical environments such in tetraalkyl ammonium ions.[1] In solids, nitrogen-14 quadrupole interactions can be characterized in the absence of external magnetic fields by nuclear quadrupole resonance (NQR).[2, 3] In a static magnetic field, the strong quadrupole interaction between the nuclear quadrupole moment of a spin I=1 and the electric field gradient (EFG) at the site of the nucleus leads to very broad spectra (up to a few MHz) that are difficult to excite and observe.[4-8] The interaction of a nucleus with a quadrupole moment Q with the electric field gradient V at the site of the nucleus can be characterized by the quadrupole coupling constant CQ=eQVZZlh and the asymmetry parameter ηQ=(VXX−VYY)/VZZ, where the principal components of the electric field gradient tensor are ordered VZZ>VXX>VYY. The asymmetry parameters ηQ can cover the full range 0<ηQ<1.
Early examples of direct detection of 14N NMR used single crystals,[4-8] where one obtains a doublet for each 14N site, with reasonably narrow spectral lines (on the order of 1.5 kHz for CQ≈3-4 MHz), provided that there are no significant crystal imperfections. The signals are spread over spectral ranges of several MHz, requiring very broad band-widths (or re-tuning of the circuits) for both excitation and signal acquisition. In static polycrystalline powders, the 14N signal intensity is spread over many MHz, so that it is difficult to recognize the singularities of the powder patterns. When the powders are spun at the magic angle, one observes families of spinning sidebands. Although for moderate spinning frequencies the signal intensity can be spread over hundreds of sidebands, quadrupole couplings up to CQ=1.5 MHz have been observed by direct 14N MAS.[9-13] Not surprisingly, the envelopes of the spinning sidebands depend not only on the accurate adjustment of the magic angle, but also on the amplitude of the 14N radio-frequency pulses and on the bandwidth of the probe and receiver systems.
The frequencies of double-quantum transitions between the |m=+1> and |m=−1> levels in I=1 systems are not affected by first-order quadrupolar interactions, as shown for 2H NMR by Pines and co-workers.[14] In combination with cross-polarization between 14N (I=1) and suitable S nuclei, one can excite 14N double-quantum coherences.[15-17] By transferring coherence back and forth between 14N double-quantum coherences and S nuclei, one can achieve indirect detection.
The 14N double-quantum transitions can also be detected directly by overtone spectroscopy, [18-22] where a radio-frequency field centered at twice the 14N Larmor frequency. This exploits the fact that double-quantum transitions are weakly allowed. Second-order quadrupole couplings, as shown in single crystals and powders, determine the line-shapes in overtone spectra.[18-22] Compounds with several 14N sites may lead to overlapping overtone patterns, which can in principle be separated by combining overtone spectroscopy with dynamic-angle spinning or with double sample rotation.[23-27]
It is possible to achieve indirect detection of 14N in spinning samples by recoupling heteronuclear dipolar interactions with a suitable spin S such as 13C.[28-30] By applying pulses at ω0N in synchronism with sample rotation, one can interfere with the averaging of heteronuclear dipolar interactions. This leads to a dephasing of the signals of the S nuclei, thus providing information on the strength of the dipolar interaction. Recoupling can also be achieved by 14N irradiation near the overtone frequency.[31, 32] By stepping the RF frequency in the vicinity of ω0N or 2ω0N, one obtains line shapes determined by first- or second-order quadrupole interactions respectively.[31] This may be regarded as continuous-wave approach to indirect detection of 14N spectra.
Another class of experiments exploits residual dipolar splitting (RDS), also known as second-order quadrupole-dipole cross term, between 14N (I=1) and S=½ nuclei such as 13C. [32-36] Since the IS dipolar interactions are not averaged out completely by magic angle spinning, because the large quadrupole coupling of the I nucleus prevents its quantization along the direction of the static field, the S resonances are split into 1:2 doublets (each component featuring a narrow powder pattern) with a splitting D, which depends on the orientation of the C-N bond with respect to the quadrupole tensor. The largest peak corresponds to the superposition of the |m=+1> and |m=−1> states of the S=1 spin, while the smaller peak corresponds to the |m=0> state. The former is further split into a doublet 2J due to the heteronuclear J coupling. Inhomogeneous broadening or relaxation may of course mask these splittings. Residual dipolar splittings have been used by Clare et al.,[29] who stepped an RF field in the vicinity of ω0N while observing two-dimensional exchange spectra of the S (13C) nuclei. If the RF field is applied for a duration comparable to the rotor period, the populations of the 14N levels are partly interchanged. As a result, the two components of the doublet in the S (13C) spectrum are swapped.[29]
It is the object of the invention to present a method for obtaining NMR spectra of quadrupolar nuclei having spin I>½ which overcomes the above mentioned problems.